Optical fibre technology is gradually making an impact on electronic systems through their ability to implement various signal processing functions, for example producing multiple delays and microwave filtering. Optical fibres also offer a low loss, compact solution to the generation of long delays for optical signals. The low dispersion properties of optical fibres effectively provide modulation frequency independent delay lines that minimise degradation of an input radio frequency signal.
Currently such delays are introduced using recirculating optical delay lines which offer a number of advantages over alternative serial or parallel type delay line architectures and provide a low cost solution for the generation of a range of delays of a pulsed signal.
From FIG. 1, a prior art recirculating optical delay line 10 comprises a fixed wavelength carrier wave optical source 11 providing a first input 12 to an external optical modulator 13 arranged to modulate the light from the optical source 11 with a pulsed radio frequency signal 14 which passes through a radio frequency amplifier 15 to act as a second input 16 to the external optical modulator 13.
A modulated optical signal 17 is generated by the external optical modulator 13 which then passes through a two-by-two optical coupler 18 that is arranged to allow 50% of the modulated signal 17 to enter a delay loop 19 and the other 50% of the modulated signal 17 to bypass the delay loop 19 and to proceed to an output of the optional coupler 18. The delay loop 19 comprises an optical amplifier 20 in series with a band pass optical filter 21 and a delay fibre 22. It will be understood that the modulated signal 17 is a series of radio frequency modulated pulses having a pulse length determined by the pulsed radio frequency signal 14.
In this delay line 10 the modulated signal 17 enters the delay loop 19 through the two-by-two optical coupler 18 and circulates through the amplifier 20, filter 21 and delay fibre 22 to achieve the desired delay duration of each pulse of the modulated signal 17. It is important to note that the pulse length of each pulse of the modulated signal 17 must be equal to or less than the overall delay duration of the delay loop 19 in order to prevent coherent optical mixing affects between overlapping sections of the same pulse of the modulated optical signal 17. The coupler 18 is also arranged to extract delayed optical pulses 23 from the delay loop 19 after each circulation of a pulse of the modulated signal 17 around the delay loop 19. Each delayed optical pulse 23 is detected by a photodiode 24 which serves to convert each delayed optical pulse 23 into an electrical signal 25 which passes through a radio frequency amplifier 26 so as to produce a delayed pulsed radio frequency output 27.
However, prior art recirculating optical delay lines 10 are constrained by the requirement that the pulses of the radio frequency modulated signal 17 must have a shorter duration than the recirculating duration of the pulse around the delay loop 19 in order to prevent coherent optical mixing effects between overlapping sections of the same pulse of the modulated signal 23 when they are detected on the photodiode 24. In some applications the pulse duration of the modulated signal 17 may be unknown or uncontrolled.